Synthesis gas is a fuel for power generation as well as an important intermediate feedstock for producing chemicals such as hydrogen, methanol, ammonia, substitute natural gas or synthetic transportation oil. Three basic types of systems and processes have been developed for the production of synthesis gas through the gasification of carbonaceous materials. They are: (1) fixed-bed gasification, (2) fluidized-bed gasification, and (3) suspension or entrainment gasification.
The hot raw synthesis gas exiting the gasifier contains entrained particulate matter comprising char, ash, and unconverted carbonaceous feedstock. This entrained particulate matter must be removed prior to further treatment of the synthesis gas, and is separated from the raw synthesis gas by a particulate removal system. The recovered particulate matter is then often recycled back to the first stage of the gasification reactor to increase overall carbon conversion efficiency.
A particulate removal system commonly comprises an initial separating device (typically a cyclone) and a downstream particulate filtration device. A cyclone creates a vortex of gas that facilitates the removal of a large percentage of the entrained particulate matter. Any remaining entrained particulates are removed from the gas stream by the downstream particulate filtration device containing numerous filter elements. These elements retain residual fine particulate matter from the gas stream while allowing passage of synthesis gas, thereby producing a particulate-free gas stream.
However, the functional efficiency of this system can be hampered by the accumulation of fine particles of char within the pores of the filter elements. These fine particles often become lodged deeply within filter element pores, which restricts the permeability of the elements to the passage of synthesis gas. Accumulation of fine particles on the elements over time leads to the formation of a “filter cake” that further decreases the permeability of the elements to gas. To remedy this condition, established practice typically involves directing a periodic pulse of high-pressure gas backwards through the filter elements (known as “back-pulsing”) in order to dislodge at least a portion of the accumulated filter cake. However, small particles that are trapped within the filter element pores are often not effectively dislodged by back-pulsing. Infiltration of these particles decreases the lifespan of the filter elements, and hampers routine operation of the particulate filtration device. Thus, there is a need to develop technology that can prevent infiltration of char particles too deeply into the pores of filter elements, thereby 1) reducing the back-pulse gas pressure needed to effectively dislodge filter cake from the filter elements, 2) slowing the rate of increase in differential pressure across the filter elements (thereby extending filter element lifespan), and 4) improving the overall operational efficiency of the particle filtration system.
Filter elements must be periodically replaced, and due to the relatively large pore size of the filter elements, new filters typically have an increased permeability to char particles. Consequently, common practice immediately following filter element replacement is to reduce the flow of raw syngas entering the filtration device until a conditioning of the filters occurs. This conditioning process typically involves the deposition of a thin layer of char particles onto, and within, the pores of the new filter elements. This layer of char particles effectively decreases the permeability of the filter elements to subsequent char particles, while maintaining permeability to gas. Current conditioning protocols require a decrease in the input rate of raw synthesis gas to the filtration unit until this initial thin layer of filter cake accumulates, and this results in decreased operating availability of the particle filtration unit. Thus, there is need to develop technology that decreases the amount of time required to achieve proper conditioning of newly-installed filter elements.
Addition of mineral fluxing agents is common in entrained flow gasification, as the addition of certain minerals to the gasification reactor reduces the fusion temperature of the ash generated by the gasification of carbonaceous material. A reduction in the ash fusion temperature decreases the viscosity of the mineral slag formed during gasification, thereby preventing plugging of the taphole that allows removal of the molten slag from the gasifier. Common practice is to mix fluxing agent with the carbonaceous feedstock prior to adding the feedstock to the gasifier. However, this reduces the overall efficiency of the gasification process both by requiring a mixing step, and also by reducing the maximum rate at which carbonaceous feedstock can be added to the gasifier. Thus, a need exists for improved methods of introducing fluxing agent into the gasification system that does not require premixing of fluxing agent with the carbonaceous feedstock prior to adding feedstock to the gasification reactor.